1. Field of the Invention
This invention relates to processes for purifying or separating liquids and particularly relates to processes involving heating or cooling for separating specific liquids from a mixture. It especially relates to cryogenic fractional crystallization and the application thereof to four-carbon mixtures including 1,3-butadiene which may hereinafter be referred to simply as butadiene.
2. Review of the Prior Art
In recent years, hydrocarbon mixtures containing n-butenes and isobutylenes have been secondarily produced in great quantities. An example thereof is the C.sub.4 distillation fraction from the refining of petroleum naphtha. In this C.sub.4 fraction, the constituents include isobutane, n-butane, isobutylene, 1-butene, cis-2-butene, trans-2-butene, and 1,3-butadiene. These are all very similar to one another in many of their physical and chemical properties. Accordingly, separation and purification of these constituents by conventional distillation procedures are difficult so that producing four-carbon materials of high purity is fairly expensive. Nevertheless, there are numerous processes which require highly pure four-carbon compounds as a raw material, such as the production of 1,3-butadiene from n-butenes and the production of methacrolein or methacrylonitrile from isobutylene. Clearly, a method for isolating one or more specific compounds that is based upon other properties than their boiling points is highly desirable.
One possibility is to utilize the freezing points of the components of a C.sub.4 hydrocarbon mixture because n-butane melts at -135.degree. C., isobutane melts at -145.degree. C., butene-1 melts at -185.degree. C., cis-2-butene melts at -138.degree. C., trans-2-butene melts at -106.degree. C., and 1,3-butadiene melts at -109.degree. C. No application of such separation by melting points is known, however, although U.S. Pat. No. 2,622,115 discloses a process for purifying binary mixtures by fractional crystallization, mentioning that cis and trans-2-butene are an eutectic pair and indicating that either component can theoretically be removed from the mixture in a pure state. U.S. Pat. No. 2,632,314 describes a fractional crystallization apparatus.
U.S. Pat. No. 2,912,469 describes a fractional crystallization process which includes feeding an immiscible liquid with the multi-component composition.
U.S. Pat. No. 3,077,744 discloses a multi-stage fractional-freezing process for separating a highly purified isoprene fraction from a mixture of C.sub.5 hydrocarbons. This process utilizes a series of low-temperature exchange coolers in which the wall film of solids is scraped away by rotating scrapers having spring-loaded blades to form a solids-liquid mixture which flows to a solids-liquid separator (such as a filter or centrifuge). Solids are removed, and liquid is sent to the next cooler. The final product contains about 70% isoprene. The process thus utilizes normal freezing of a liquid by contact with a cold surface.
U.S. Pat. No. 3,264,363 discusses purification of five-carbon mixtures with liquid methane by direct contact of the methane with the mixtures so that it is an extraction type of process rather than a fractional crystallization process.
Crystallization processes from the melt utilize differences in melting points of the components in a mixture. According to an article in Hydrocarbon Processing, Dec. 19, 1966, pages 97-102 by John E. Powers, commercial separation and purification processes using crystallization from the melt can be grouped according to three basic procedural approaches: normal freezing, zone refining, and column crystallization in which there is differential countercurrent contacting of crystals and melts.
Zone refining or zone melting has had considerable usage in the production of high-purity materials for semiconductors, but this procedure has been limited as to size of equipment because the solid-liquid interfaces forming the zone are distorted by heat transfer from natural convection. Consequently, even though some organic materials of high purity have been produced by zone refining on a commercial basis, wide-spread application thereof has been significantly hampered.
Column crystallization appears to be well suited to commercial scale processing for hydrocarbons such as C.sub.4 mixtures. In a column crystallizer, solid and melt are moved counter-currently in intimate contact so that there is adequate reuse of energy of crystallization and the possibility of operating a single crystallization unit under steady-state, continuous flow conditions without handling any solids externally of the unit. Moreover, the products from column crystallizers are frequently in excess of 98 weight percent purity after a single pass through the column. Column crystallizers can be center fed or end fed. A suitable center-fed column crystallizer is provided with a spiral-type conveyor, a freezing section at its upper end, and a melting section at its lower end. An end-fed column crystallizer is suitably provided with a melting section at its lower end and a chiller at its feed end to provide crystals to be packed into a bed above the melter with a circumannular filter surrounding the upper end of the bed for removing mother liquor therefrom. The high-purity product emerges from the bottom beneath the melter.
In order to consider the utilization of fractional crystallizers for processing the large quantities of C.sub.4 mixtures that are available from the petroleum industry at the present time, it is necessary to have available large quantities of refrigeration. However, there are numerous cryogenic processes which can supply the low-temperature refrigeration which is needed, such as hydrocarbon cascade systems, multistage expansion systems, and the like.
Excessive energy usage will not occur if adequate insulation is used and efficient heat exchangers are installed. High vacuum insulation, multiple-layer vacuum insulation, and evacuated powder insulation are preferred, particularly with a vacuum of at least 10.sup.-4 mm Hg.
Moreover, low-level refrigeration can be obtained by vapor-compression refrigeration of the BB stream, using propane or ammonia for condensing and discharging the heat therein to cooling water. The BB stream is preferably at least pre-cooled by such a system for refrigerated storage or for direct combination with a cryogenic process.
In certain coastal areas, particularly where deep-water port facilities are located close to an oil refinery, there are receiving terminals for vaporizing liquefied natural gas (LNG), which is being shipped on a large scale in ocean-going tankers, for use by industrial and private consumers. In these terminals, LNG at -260.degree. F. (-162.degree. C.) is converted to the vapor state, usually around ambient temperatures. As discussed in Chemical Engineering Progress, Volume 68, Number 9 for September 1972, much heat transfer equipment is needed for both base load LNG plants and for peak shaving LNG plants.
The type of equipment for such vaporization is dictated in many cases by environmental requirements as well as by available heating media. Some of the systems which have been suggested and put into use are submerged combustion systems, intermediate fluid systems in which another fluid is heated and then used to evaporate the LNG, and direct systems in which there is a heat exchange between the LNG and air or water. When using water from rivers or oceans, an open rack exchanger is quite effective.
If fuel is burned, i.e., using LNG as the fuel, in a submerged combustion system or in intermediate fluid systems, the fuel consumption is 1.5-2% of the LNG, but this type of unit is frequently selected because its capital cost is low. An open rack exchanger may cost three or four times as much, but its operating cost may be 1/5 to as little as 1.10 of the combustion system.
Moreover, the amount of heat that is needed for vaporizing the immense quantities of LNG being shipped is not inconsequential. For example, a one-billion standard-cubic-foot/day receiving terminal requires water on the order of 200,000 gallons per minute and uses 6-foot to 8-foot diameter water mains. There are environmental requirements which must also be considered with respect to thermal pollution and other ecological impacts. Therefore, some means for combining the need for refrigeration in a fractional crystallization process and the need for heat in an LNG vaporization process is highly desirable.